1. Field of the Invention
The present invention relates to image reading apparatuses, methods of reading images, whiteness level correcting methods and whiteness level generating methods, and more particularly to an image reading apparatus and an image reading method for reading an image on an original, and to a whiteness level correcting method and a whiteness level generating method employed in reading an image on an original.
Recently, with less paper being used in offices, image inputting products (image scanners) have been used as inputting means for storing documents in optical disks or the like. As the volume of images increases, high-speed input systems are used more often. Hence, use of automatic paper feeding is being practiced more and more widely.
In the case where automatic paper feeding is used in image reading, a CCD (charge coupled device) starts a reading operation at a member (hereinafter, referred to as a backing part or a backing member) for supporting the backside of an original so that the leading edge of the original can be read.
Since a reading is started at the backing part, a ground color (reference color) of the original cannot be read properly. In some cases, an image reading apparatus may produce a darkened output. Another problem in a conventional image reading apparatus is that, when a large volume of input originals are processed, the backing part and the input original may undergo friction, resulting in a flaw extending in a longitudinal direction being created in he backing part. When the flaw is created, the output f the image reading apparatus often accompanies a white or black streak.
Thus, an arrangement, whereby it is ensured that the leading edge of an automatically fed original is read, is met by a difficulty in obtaining a high-quality image which is true to the original.
For this reason, there is required an image reading apparatus capable of producing a high-quality image true to the original that is fed automatically.
2. Prior Art
FIG. 1 shows a construction of a conventional image reading apparatus 91. The image reading apparatus 91 has an electrooptical converter 92 for reading an image optically, an image processing unit 93 for processing a signal obtained in the electrooptical converter 92, a feeding mechanism as for transporting an original 94, a carriage driving mechanism 96 for moving the electrooptical converter 92, and a controller 97 for controlling the image processing unit 93, the feeding mechanism 95 and the carriage driving mechanism 96.
The image reading apparatus 91 includes: a flat-bet (FB) function whereby the electrooptical converter 92 is moved by the carriage driving mechanism 96 to scan an original 94 so that the image thereon is read; and an automatic document feeder (ADF) function whereby the image is read while the original 94 is being moved by the feeding mechanism 95 and scanned.
As shown in FIG. 1, if the reading is executed in an FB mode, the controller 97 controls, upon power ON of the apparatus, the carriage driving mechanism 96 so that the electrooptical converter 92 is moved in a direction indicated by an arrow A.sub.1 to reach a home position P.sub.0 and stand by for an instruction signal. An original cover 98 is opened so that the original 94 is set in a platform 99 for an original. When an instruction to read an image arrives, a fluorescent lamp provided in the electrooptical converter 92 is turned on, whereupon the electrooptical converter 92 is further moved in the A.sub.1 direction to reach a position P.sub.1 so as to scan a reference plate 100 and read a reference whiteness level. Subsequently, the electrooptical converter 92 scans the original 94 so as to start reading the image thereon at a predetermined position P.sub.2. When the reading is completed, the electrooptical converter 92 is returned to the home position P.sub.0 and stands by for a next instruction. In reading the original 94, the reference whiteness level is used as an initial value, and other whiteness levels are successively made to follow the initial value. In this way, correction is made so that proper contrast is obtained.
If the reading is executed in an ADF mode, the controller 97 controls, upon power ON of the apparatus, the carriage driving mechanism 96 so that the electrooptical converter 92 is moved in the A.sub.1 direction to reach a home position P.sub.0 in the left half of the apparatus in FIG. 1 and stands by for an instruction signal. The original 94 is set in a shooter 101. When an instruction to read an image arrives, the fluorescent lamp in the electrooptical converter 92 is turned on, and the electrooptical converter 92 is further moved in the A.sub.1 direction to reach the position P.sub.1 and read the reference whiteness level of the reference plate 100. Subsequently, the electrooptical converter 92 is moved to an ADF reading position P.sub.3 in the left half of the apparatus in FIG. 1 and stands by for an instruction. The feeding mechanism 95 is then driven so as to draw the originals 94 mounted in the shooter 101 one by one into the ADF. Reading is started from a predetermined position. The document 94 read is ejected to a stacker 102.
The original 94 is guided by a backing part 103 so as to face directly opposite the electrooptical converter 92.
FIG. 2 shows a conventional backing part 103. In ADF reading, in order to ensure that a leading edge 94a of the original 94 is read, reading is started before the original 94 is transported to a reading position. The backing part 103 is read first, whereupon the original 94 is transported and read. Conventionally, the entire backing part 103 is painted in white in an assumption that the ground color of a commonly used original 94 is white.
The image processing unit 93 converts an analog electric signal from the electrooptical converter 92 into a digital signal on the basis of the reference whiteness level. In order to prevent data from being ruined by a level of the analog electric signal becoming higher in brightness level than the reference whiteness level, a conventional apparatus is equipped with a circuit whereby the electric signal is compared with the whiteness level required for conversion of the analog electric signal into the digital data. When it is determined that a difference between the analog electric signal and the whiteness level exceeds a predetermined level, the analog electric signal is made to indicate the whiteness level to make sure that the ground color of the original is white. Since image reading is started at the backing part 103 in the ADF, the color (white) of the backing part 103 is read first, whereupon other whiteness levels are made to follow. That is, a white-level following operation is started at the leading edge of the original.
It is to be noted that an original read by an image reading apparatus can be an original characterized by a contrast between the ground color of the original and a line drawing etc. on the original, or an original like a photograph whose image representation does not depend on the ground color.
The original which contains a line drawing etc. does not necessarily have the ground color of white. Hence, in order to obtain optimal contrast, a white-level following operation for determining the whiteness level should be carried out in accordance with a read signal. A reading mode wherein a reading is executed while whiteness levels are made to follow the ground color is referred to as a line art mode.
An image on an original such as a photograph is represented without the ground color being shown. An image true to the original cannot be obtained if a white-level following operation is conducted on an original such as a photograph. In this type of original, an image is read such that the reference whiteness level is determined by employing white as the fixed reference color. An image reading mode wherein the reference whiteness level is determined by employing white as the fixed reference color is referred to as a photograph mode.
Thus, the conventional image reading apparatus is constructed such that an image reading mode (line art mode or photograph mode) adapted for the type of original (a line drawing or a photograph) is chosen for image reading suitable for the original to be carried out.
It is to be noted that, irrespective of whether the line art mode or the photograph mode is employed, there is provided only one (white) backing part in the conventional image reading apparatus. In the line art mode wherein reading (white-level following operation) is started at the backing part in order to ensure that the leading edge of the original is read, the color (white) of the backing part is stored as the ground color of the original, and shading correction is conducted accordingly. There is a problem in that, when the original has a darkened color, a gap is created between the reference white level data and the data for the ground color, with the result that an accurate white-level following operation cannot be performed, and the image becomes darkened.
It is also to be noted that the white-level following operation of an original is conventionally conducted at the backing part or at a leading edge (which has a width of about 3 mm) of the original. A flaw in the backing part may cause the reflectance thereof to vary, with the result that different data may be output even when same reference plate is read. Consequently, reading correction is conducted even when data relating to the ground color vary depending on CCD pixels, with the result that a normal white-level following operation cannot be conducted, and a streak may be created in the image.